During FY14 we accomplished the following: 1. Completed studies on the step-wise establishment of tissue-specific chromosome conformation of the IgH locus. Specifically we found the CTCF-dependent locus compacting interactions did not require Pax5 or E. However E-dependent long-range interactions required both YY1 and Pax5. All loop formation occurred only in B lineage cells. These studies were carried out in collaboration Dr. Ann Feeney (The Scripps Research Institute, San Diego). 2. We developed the use of 3-4kb FISH probes for high resolution FISH studies of chromosome conformation. We used these probes to study inter-chromosomal interaction between IgH and c-Myc loci on chromosomes 12 and 15, respectively. 3. We assayed IgH locus conformation in primary bone marrow pro-B cells from old mice. Preliminary experiments indicate severely attenuated locus compaction in old pro-B cells. Such de-compaction may lead to reduced efficiency of VDJ recombination with age, and result in a smaller repertoire of B cells available for effective immunity.